1. Field of the Invention
The present invention relates to a master cylinder of a brake system in an automobile and, more particularly, to a tandem master cylinder configured to have the braking power increasing ratio between the front wheels and the rear wheels follow a nonlinear curve for improving the braking efficiency of the automobile.
2. Description of the Prior Art
Nowadays, most passenger cars using a hydraulic brake system adopts safety devices such as a dual brake system and an anti-skid brake system (ABS). Also, the dual brake system adopts a tandem master cylinder, a safety cylinder and a pressure differential warning valve. As is well known, the tandem master cylinder has two hydraulic cylinders coupled in series in order to divide the hydraulic circuit of brake system into front and rear wheel sides.
A conventional tandem master cylinder is represented by FIG. 1, having two hydraulic cylinders coupled in series. The device comprises a first chamber 10A and a second chamber 10B, charging brake oil, a first piston 12A and a second piston 12B constituting one side of each chamber, and a first spring 14A and a second spring 14B disposed in each chamber. Also, a first hydraulic port 16A leading to the rear wheel cylinders is formed at the first chamber 10A and a second hydraulic port 16B leading to the front wheel cylinders is formed at the second chamber 10B. Connected at the second piston 12B is a hydraulic push rod 20 of a brake booster, which is coupled to the brake pedal 18 in a well known manner.
When the automobile driver operates the brake pedal 18, the push rod 20 pushes the second piston 12B to compress the brake oil (hereinafter called oil) and the second spring 14B in the second chamber 10B. Simultaneously, the second spring 14B urges the first piston 12A to compress the oil and the first spring 14A in the first chamber 10A. Thus, the oil in each chamber 10B and 10A is transferred from the second port 16B and the first port 16A to the front wheel cylinders and the rear wheel cylinders, respectively, through each brake line.
Midway in the rear wheel brake line, a proportioning valve 26 (hereinafter p-valve) is provided as the anti-skid brake system, which reduces the oil pressure increasing rate of the rear wheel cylinders compared with the front wheel cylinders, as the brake power is increased. This prevents the rear wheels from skidding when braking the automobile.
The p-valve 26 is not operated at the initial stage of braking, however, it is operated when the oil pressure at the hydraulic port 16A to rear wheel cylinders is increased to a predetermined level, to reduce the increasing rate of the brake power of the rear wheels.
As can be seen in the graph of FIG. 2, if brake power is continuously increased, the brake power (oil pressure) for both the front wheels and the rear wheels is increased at almost the same rate, following a linear slope to the operating point of the p-valve. After this, the increasing rate follows another linear slope which is less than the first one, due to the operation of the p-valve.
However, since the above change of oil pressure is effected between two linear slopes, and the change occurs abruptly at the operating point of the p-valve, it is found that the braking power or performance of the automobile is not effective and that frequent changes in the oil pressure weakens the mechanical parts of the p-valve as well as components of the rear wheel brake.